The present invention relates generally to operation of a radio frequency transponder system having a radio frequency reader unit and a radio frequency identification device and, more particularly, to a method of operating the radio frequency transponder system to detect the proximity of the radio frequency identification device to the radio frequency reader unit which is in a reduced power state.
A radio frequency (RF) transponder system typically comprises an RF reader unit and a radio frequency identification (RFID) device. The RFID device is commonly termed an RFID tag. Operation of the RF transponder system is generally characterized by multiple operating modes including excitation, response and read modes. The RF transponder system requires electrical power to operate in each of these modes. In particular, the RF reader unit must be powered during the excitation and read modes while the RFID device must be powered during the response mode. In many conventional RF transponder systems the RFID device is a passive device, i.e., the RFID device lacks an internal power source or physical connection to an external power source. The passive RFID device is powered remotely by the RF reader unit while the RFID device is free of contact with the RF reader unit. An exemplary passive RFID device and its method of operation are disclosed in U.S. Pat. No. 4,730,188 to Milheiser. The RF reader unit is conventionally connected to an electrical power source, such as an ac power line, which powers the RF reader unit.
The present invention has identified the need for an RF reader unit, which is powered by a self-contained portable power source integral with the RF reader unit, such as a small disposable or rechargeable battery. This enables the user to position the RF reader unit in a remote location which lacks access to an ac power line or an ac power outlet. A battery, however, has a finite life necessitating replacement of the battery in the RF reader unit at the end of its useful life, which is both costly and time consuming. Accordingly, it is desirable to reduce the power demands on the battery of the RF reader unit during operation of the RF transponder system, thereby extending the useful life of the battery.
As such, a need exists for an effective method of operating an RF transponder system with a reduced electrical power demand. Accordingly, it is an object of the present invention to provide an RF transponder system operable at a reduced electrical power state. More particularly, it is an object of the present invention to provide a method of operating an RF transponder system in a power conserving manner, wherein the system transitions between a reduced power state and an increased power state as a function of the specific operating mode of the system. Still more particularly, it is an object of the present invention to provide such a method, wherein the RF transponder system has an effective RFID device detection mode of operation at a reduced power state and has excitation, response and read modes of operation at an increased power state. It is another object of the present method to provide such a method, wherein the power requirements of the RF transponder system are fully satisfied by a disposable or rechargeable battery residing in the RF reader unit of the system.
These objects and others are accomplished in accordance with the invention described hereafter.
The present invention is a method of operating an RF transponder system comprising an RF reader unit and a passive RFID device. The RF reader unit includes an excitation signal generator circuit, an excitation mode activation circuit coupled to the excitation signal generator circuit, an RFID device detection circuit coupled to the excitation mode activation circuit, and a power source for powering the electrical components of the RF reader unit. The excitation signal generator circuit is operable in either a reduced power state or an increased power state. When operating in the reduced power state, the excitation signal generator circuit enables generation of a plurality of ring signals which exhibit an RFID device detection parameter. When operating in the increased power state, the excitation signal generator circuit enables generation of an RF excitation signal which powers the RFID device. The excitation mode activation circuit, which is preferably a logic flip/flop switch, enables switching of the excitation signal generator circuit between the reduced power state and the increased power state in response to the RFID device detection parameter passing a variation threshold level. The RFID device detection circuit is operable in the reduced power state to determine when the RFID device detection parameter passes the variation threshold level. The power source is in the form of a small portable battery which provides reduced electrical current to the excitation signal generator circuit in the reduced power state and increased electrical current to the excitation signal generator circuit in the increased power state. The RFID device includes a transponder circuit which causes the RFID device detection parameter of the ring signals to pass the variation threshold level when the RFID device is positioned in a proximal space relative to the RF reader unit.
The present invention is more particularly a method of operating the RF transponder system to detect the presence of the RFID device in the proximal space of the RF reader unit. The method is initiated with the excitation signal generator circuit of the RF reader unit operating in a reduced power state which is effected by drawing reduced electrical current from the power source. The excitation signal generator circuit generates the ring signals in response to the reduced electrical current and transmits the ring signals into the proximal space. The RFID device detection circuit of the RF reader unit, which is in electrical communication with the excitation signal generator circuit, receives and evaluates the ring signals to determine variations in the RFID device detection parameter of the ring signals, such as variations in the decay rate or average voltage of the ring signals. When the variation in the RFID device detection parameter passes the variation threshold level due to the presence of the RFID device in the proximal space, the excitation mode activation circuit transitions the excitation signal generator circuit from the reduced power state to the increased power state and generation of the ring signals is terminated.
The excitation signal generator circuit draws increased electrical current from the power source in the increased power state to generate the RF excitation signal. The RF excitation signal is transmitted to the RFID device positioned in the proximal space, powering the transponder circuit of the RFID device. The ring signals and the RF excitation signal have substantially the same frequency which is substantially equal to the tuned frequency of the transponder circuit and the excitation signal generator circuit. The transponder circuit processes the RF excitation signal, generates an RF response signal in response to the RF excitation signal, and transmits the RF response signal to an ER circuit housed in the RF reader unit. The ER circuit, which includes the excitation signal generator and RFID device detection circuits, reads the RF response signal upon receipt. After the RF response signal is read; the excitation signal generator circuit is transitioned back to the reduced power state and generation of the ring signals resumes while generation of the RF excitation signal is terminated.
It is noted that the duty cycle of the excitation signal generator circuit is substantially lower when operating in the reduced power state than when operating in the increased power state. As a result, the life of the power source is substantially extended and more electrical power is available to the other operations of the RF transponder system.
The present invention will be further understood from the drawings and the following detailed description.